Semiconductor integrated circuits often require a voltage supply circuit or voltage "reference" for providing a predetermined voltage level. The voltage level provided by such a reference circuit undesirably tends to fluctuate during operation because of temperature variations in the underlying semiconductor body in which the circuit is integrated. However, in the semiconductor art of analog-to-digital and digital-to-analog converter circuits, for example, a voltage reference is desirable which does not fluctuate in voltage level by more than typically about 0.005 volts or less. Therefore, steps must be taken to stabilize the reference circuit against temperature fluctuations.
In order to obtain a stable reference in either bipolar or complementary MOS (C-MOS) technology, the industry generally uses voltage references utilizing either the voltages associated with reverse breakdown phenomena in Zener diodes or the voltages provided by bandgap reference circuits. Such bandgap reference circuits are described, for example, in Analysis and Design of Analog Integrated Circuits, Paul R. Gray and Robert G. Meyer, at pp. 248-261. In N-MOS technology, which uses a P-type semiconductor substrate, none of the above mentioned voltage references is feasible. More specifically, Zener diode reverse breakdown cannot easily be used because all PN junctions are designed to withstand the highest possible reverse voltage available on the semiconductor chip in which the circuits are all integrated; hence these junctions cannot readily be driven into reverse breakdown. Moreover, known bandgap reference circuits cannot easily be used since they require constantly forward biased junctions; but, since the P-type substrate in N-MOS integrated circuits is connected to the most negative potential in the system, the requisite constantly forward biased junctions cannot readily occur. Thus, to implement either reverse breakdown Zener or bandgap reference circuits in N-MOS technology would require additional costly fabrication steps, which would impair the economic advantage in N-MOS technology.
It would therefore be desirable to have a voltage reference circuit which can readily be fabricated in N-MOS technology.